Heavy oil recovery



Aug. 25, 1959 F. E. cAMPloN ETAL 2,901,043

HEAVY OIL RECOVERY Filed July 29, 1955 2 Sheets-Sheet 1 F 'G' 2INVENTORS FRANK E. CAMPON EUGENE D. GLASS RICHARD A. MORSE BYW W ATTok/v Y Aug. 25, 1959 Filed July 29, 1955 F. E. CAMPION ETAL 2,901,043

HEAVY on. RECOVERY 2 Sheets-Sheet 2 3.0 l i f i TEMPERATURE OF FRACTUREAT VARIOUS TIMES i RELATIVE TO PASSAGE OF COMBUSTION ZONE i PAST A GIVENPOINT AT A VELOCITY OF ONE i FOOT PER DAY. 2.o .I n: D u |.0 z 2 I... mD m E O U o l l l I l I v I I 4 '-2 O 2 4 6 B IO I2 I4 I6 I8 2O 22 24TIME IN DAYS FIG. 3

TEMPERATURE DISTRIBUTIONS VERTICALLY AWAY FROM FROM FRACTURE AT VARIOUSTIMES RELATIVE TO PASSAGE 0F COMBUSTION ZONE 26 AT VELOCITY OF ONE FOOTPER DAY.

O DAYS IO I2 I4 DISTANCE FROM FRACTURE IN FEET FIG.4

IN VEN TORS FRANK E. CAMPION EUGENE D, GLASS RICHARD A. MORSE ATTOR EYUnited States Patent HEAVY OIL RECOVERY Frank E. Campion and Eugene D.Glass, Tulsa, Okla;, and Richard A. Morse, Pittsburgh, Pa., assignors toPan American Petroleum Corporation, a corporation of DelawareApplication July 29,` 1955', Serial No. 525,284

4 Claims. (Cl. 166-11) This invention relates to the recovery of oil orhydrocarbons from the underground strata in which they occur and isdirected'particularly to the recovery of heavy oils and similarcarbonaceous substances byapplying heat to them. More specically, theinvention is directed to a process or method of heavy oil recovery byheating wherein the heat is generated byv combustion or oxidation withinthe deposit or reservoir stratum itself.

There are many'known fields where the percentage of oil recovery isquite low. This may be due-to one or a combination of several causes. Insome fields, the poor recovery of oil is'due mostly to unfavorableconditions of porosity and/or permeability of the reservoir medium. Inother fields it is known that these latter conditions are favorable, andthe poor recovery of oil is due to the characteristics of the oilitself, such as high viscosity, low gas saturation, or high Wax content.It is in the latter type of. reservoir that our invention is most usefulfor improving recovery, but its utility is not necessarily limited tosuch ields.`

Overa substantial period of years, many suggestions have been made forimproving recovery by applying heat to make. oil more/easily recoverablein va-rious ways, such as by increasing vapor pressure or reducingviscosity. Some of these suggestions have'been tried with varyingdegrees of success. The usefulnessof many of these methods orsuggestions has been limited in a number of ways. For example, sometypes of reo'overyfopera'- tions may require injection pressuresthat-are so high as to be uneconomical; the required volumes of gas tobe injected may be excessively large `per barrel of liquid recovered;the minimum. temperatures which can be maintained may still be higherthan are desirableV or necessary; more of the hydrocarbonsin place may.be consumed than is necessary or. desirable; heat losses may beexcessive; only Ione pass through the reservoir formation can be made,and if this is unsuccessful, it cannot be'repeated; the progress of theheat-generating combustion front'or Wave maybe diicult to vary orcontrol; the tarea of operation of the recovery process may be aconstantly expanding one, ultimately requiring very large capacities ofenergy input; and oxygen enrichment may be required at additionalexpense.

The primary object of our invention accordingly is to provide, for therecovery of heavy oils, tar, and like carbonaceous substances, a processor method which uu'- lizes heat generated by, combustion within theproducing forma-tion in a way ormanner which avoids or overcomes most ofthe foregoing mentioned diiculties or disadvantages. More specic objectsof our invention are to provide an underground combustion process bywhich: (l) a controllable andlimited amountof heat is generated withintheL formations, so as toobtain any` desired degree ofviscosity-reduction and otheraecompany.` ing effects, withoutsimultaneously causingoverhea-ting, of the formation in places or`overheating: and cracking of the bulk-of the in-plaee hydrocarbons; (2)extraneous fuel may be. utilized.v tor any desi-red; extent 2,901,043Patented Aug. 25, 1959 ICC to conserve the hydrocarbons it is desired torecover; ('3)` more thanone pass through a given formation can be madewhenever desired; (4) the generation of heat acts a's-l an aid to moreconventional methods of recovery; (5)- a fractional yrecoveryV can beeffected' to recover only the most volatile, and often the mostvaluable, fractions of the oil in'place, leaving behind only the leastdesirable heavy constituents; (6) if necessary or desirable, thegeneration of heat by combustion can be ldiscontinued at any time andrestarted or repeated as desired at some later time; (7) recovery can becarriedout'completely in any- Ismall portion of a large tield withoutaffecting the remainder of the eld and Withoutneed for a'large plantinvestment to supply an everincreasing amount of input energy. Other andfurther objects, uses, and advantages'of the invention will becomeapparent as the description proceeds.

Stated ina most brief fashion, theforegoing and other objectsV areaccomplished by our invention by performing what may be designatedasbackward or'countercurrent'combustion in fractures.

Inconventional recovery processes, involving Ithe injection-l of adrive-producing medium through a firs-t' well and recovery of theproducts from a second wellspaced at some distance from the first, it isordinarily considered-"a disadvantage tohave highly permeable streaks orstrata extending between the two wells. These are thought to causechanneling of the injected fluids, which results-'inthebypassing ofportions of the oil in place. Thus, efforts are ordinarily. made toequalize the formation` permeability by closing off the most permeablestreaks, leaving a substantially uniform but somewhat loweroverall valueof permeability of the formation to be subjected to the recoveryprocess.

In our invention, on the other hand, such highly permeable horizontalstreaks or channels as are normally closed on arenot only necessary,but, if they are not naturally present at the appropriate places, theymust be articially created. Our invention accordingly comprisesproviding at least one input well and one or more output wells spacedtherefr-om` in -a producing horizon, which wells are connected Vbyeither naturally occurring or artiflcially induced horizontal fractureswithin the producing formation. Combustion is initiated at the bore ofthe output well inany suitable manner. This combustion is supported bysupplying :an oxygen-containing gas, preferably air, alone or in acombustible mixture along with a fuel gas, preferably natural gas. Theoxygencontaining gas -or mixture is injected through the input well andflows through the fracture or fracturesto the initiated zone ofcombustion .at the bore of the output well. This mode of oxygen supplycauses the combustion `zone or zones to move slowly through thefractures awayfrom the output well bore toward'the source of oxygen atthe input Well.`

During this movement, the heat generated by combustion occurring chieflywithin the fractures diffuses verticall-yithrough :the reservoir stratumby conduction through the reservoir rock.- The heavy oil or othercarbonaceous material in the rock pore spaces has its viscosity lowered,its vapor pressure increased, and/or one or more of several 1 additional`effects `may occur.

By gravity oW, the reduced viscosity oil or hydrocarbon-may enter thefractures and be driven to the output well by the ow of combustionproduct gases through the fractures; The heat diffusing throughthe rockmay cause evolution of-gases from, Ior vaporization of light componentsof, the -oil within the rock pore spaces, thereby forcing vthe oil youtof the pore spaces and into the permeable fractures. Also, the heatingmay be suicient to vaporize the connate water normally present Withinmost formations, thus subjecting the oil and reservoir rock to a kind ofsteam-pressure drive and stripping action. Also, the viscosity of theoil may be lowered to the point where the pressure of gas injectedwithin the reservoir rock itself can drive the oil to the output well.Regardless of which one `or what combination of these mechanisms isbrought into play, the net result of reservoir heating in the mannerdescribed, by diffusion of heat from zones of combustion withinfractures, is a substantial increase in the percentage of oil recovery.

This will be better understood by reference to the accompanying drawingsforming a part of this application and showing diagrammatically andgraphically an ernbodiment of the invention and the relationships ofcertain variables. In these drawings,

Figure 1 is a diagrammatic cross-sectional view through a section of theearth between two wells which penetrate an oil-bearing stratum to besubjected to recovery according to the invention;

Figure 2 shows logs of varying temperature distributions through theproducing and adjacent strata;

Figure 3 is a typical graph of the temperature variation with time at apoint within a fracture before and after passage of a combustion zone;and

Figure 4 shows graphs of typical temperature variations as a function ofdistance from a fracture, at various times relative to the passage of acombustion zone through the fracture.

Referring now to these drawings in detail and particularly to Figure 1thereof, an oil-bearing stratum 10 is shown diagrammatically in crosssection as penetrated by two spaced wells 11 and 12, which are drilledfrom the ground surface 13 through the stratum 10. Extendinghorizontally between the Wells 11 and 12 through the stratum are aplurality of horizontal fractures 15, 16, and 17. Besides extendingbetween the wells 11 and 12 as shown on the drawing, it is to beunderstood that the fractures 15, 16, and 17 normally extend in alldirections around the various wells penetrating stratum 10,

of hydraulic fracturing well known in the art, or they may be naturallyoccurring streaks of high permeability.

The vertical spacing or depth interval of the fractures within thestratum 10 is a matter of some importance. Preferably the fractures arespaced approximately uniformly apart in depth. As in a majority of casesnaturally occurring fractures or permeable streaks will not be presentat the desired levels within the stratum 10, a usual preliminary step inrecovering oil by our process comprises artilicially creating fracturesat the desired depths between and around the wells 11 and 12. Forexample, by hydraulic fracturing, horizontal channels are created aroundthe well 11 extending to distances at least as great as the bore of well12. These same fractures can then be enlarged and extended furtheraround well 12 by injecting fracturing media into it. In a similar way,other wells not shown can be connected into the original set offractures created between the two illustrated wells 11 and 12. Also, ifdesired, the capacity of natural fractures may be enlarged by causingone or more of the articial fractures to pass along them.

The vertical positions of the fractures 15, 16, and 17 with respect tothe boundaries of the stratum 16 are also of importance. From theviewpoint of maximum utilization of heat, a single fracture passinghorizontally through the center of the stratum would be the mostefficient, but this would serve as a gravity drainage flow channel foronly the upper half of the formation 10.

For most efficient recovery by gravity drainage, a single channel at thelower formation boundary would be best, but this would involve a loss ofabout half of the generated heat to the underlying strata. In practice,these two conflicting requirements are compromised by using two or morefractures simultaneously, one of which is preferably near the bottom ofthe stratum, such as is exemplified by the fracture 17, while the otheror others are spaced approximately uniformly throughout the rest of theproducing stratum 10 thereabove.

With the fractures 15, 16, and 17 avaliable, the recovery of oil by theprocess of our invention is begun by initiating combustion at the boreof the output well 12. This may be done in any of several ways. Forexample, the face of the formation 10 exposed in the bore of well 12 isheated to above the ignition temperature of the combustible gas-airmixture to be burned by means of a well-bore heater. This may be eitherelectrical or one tired by a combustible mixture of gases injected downtubes inserted into well 12. During and/or immediately after the initialheating of the well bore 12, combustionsupporting, oxygen-containing gasis injected down input well 11, whence it iiows through the fractures15, 16, and 17 to the bore of well 12 where combustion within thefractures then begins. This combustion continues as long asoxygen-containing gas is supplied, while the location of the zones ofcombustion slowly moves away from well 12 through the fractures towardwell 11.

The mechanism by which this countercurrent combustion zone propagationtakes place is qualitatively simple, though somewhat difficult todescribe quantitatively. Consider a first point within a fracture wherecombustion is occurring. The heat released at this point flows byconduction through the rock in any direction where the temperature islower. Thus, the heat travels not only vertically away from the point ofcombustion, but also horizontally along the fracture in thecountercurrent direction. As a result, a second point in the rock at theboundary of the fracture in the upstream direction subsequently becomesheated to a temperature high enough to ignite the combustible gasmixture arriving there, combustion and heat release then occur at thesecond point, some heat travels to a third point still further upstream,and so on. In reality of course, all these phenomena occur smoothly andin a continuous manner, rather than in the point-to-point fashionassumed for explanation.

As combustion-supporting gases continue to be injected into well 11 andthrough the fractures to the combustion zones therein, increasinglygreater amounts of heat are released within the fractures at thelocations of the combustion zones. By conduction this heat diffusesthrough the reservoir medium to the portions of stratum 10 lying betweenand above or below the fractures. As the temperature of the oil instratum 10 near well 12 rises due to this heat transfer, the oil flowsto the well bore 12 by any of several paths. By the formation of gasbubbles within the reservoir rock medium, the oil may be forced out ofthe pore spaces and directly into well 12. By heating, the oil viscositymay be reduced to the point where it iiows readily by gravity to one ofthe fractures 15, 16, or 17, where the rapidly flowing combustionproduct gases sweep it along the fracture into well 12. Removal of oilfrom the interior of the stratum 10 may also be assisted by thevaporization of such connate water as occurs within the rock pore spacesalong with the oil, provided the temperature reaches a sufficient levelfor vaporization of the water at the reservoir pressure existing.

Not al1 of the injected oxygen or oxygen-containing gaseous mixture mayHow through the fractures, but a minor proportion of the injected gasesmay iow also through the interior of the stratum 10. Upon reaching apoint in the rock space between the fractures where the temperature ishigh enough for ignition to occur, oxidation utilizing this oxygenoccurs to augment the heat car- :existing in the, combustion zone..rari-.introduced andapaaad by. suitable intervals of, time,

'fried by difusionthrough'ithe rock lfrom the fracture cornbustionzones. The main ybullcofvgeneration of heat,

throughl the reservoir medium. Generally speaking, the

`relative amounts of heat generated within the fractures and within theformation arein about the same ratio as is `the dow of gas in thefractures compared to.that through the formations.

The composition ofthe input gases to well y11 may be varied widely. Ascompared with concurrent combustion processes,V wherein the combustionzone movement and the fluidflow through the Yformation are inthe samedirection, a particular advantage of the present invention isthat, asmuch gaseous fuel as desired may be incorporated in the input Vgasstream. The combustion zone `nevertheless continues to propagate in thedesired countercurrent direction. Thus, it is entirely possible tosupply enough natural gas, for example, as fuel in the input stream toconsume al=1 of the oxygen, thereby furnishing ally of the heat neededfor oil recovery without requiring combustion of any substantialportionof the oil in place. In fact, if desired for any reason, naturalgas could constitute a substantially larger proportion of the input gasstream than the amount required to form a stoichiometric mixture withthe oxygen. Alternatively, any part of the oxygen may be consumedby thefuel gas injected along with it, and the remainder-of the fuel necessaryto utilize the remaining oxygen can -be supplied .by the oil and gas inplace. The ultimate that can be accomplished in this direction, ofcourse, is` to supply onlyoxygen in the input gas stream, relying uponthe hydrocarbons in place `for the'entire supply of fuel.

Also, as to inert gases or iluids, the composition of the input fluidstream can be varied widely. When using air, which is to be preferred,the inerts may constitute up to about 80 percent of the inputgas streamand are principally nitrogen. This proportion may, if desired, bereduced by enriching the oxygen content of the air by any desired amountup to the use of pure oxygen, which, however, is generally noteconomical; or the proportion may lbe increased by adding inert gasessuch as flue gases, additional nitrogen, water vapor, and the like.

The amount of or ratio of inert gases or fluids to oxygen in the inputgas stream forms an important manner of regulating the speed ofpropagation of the fracture combustion zone. For a given rate of oxygensupply, it is apparent that increasing the proportion of inert gases`wil-l decrease the combustion zone propagation rate, as the concurrentheat transfer by the inert gases acts in opposition to thecountercurrent heat transfer by conduction through the reservoir rock.The converse of this is also true. Decreasing the proportion of inertgases or fluids in the input gas stream allows the propagation to take`place more nearly at the speed of heat transfer through therock byconduction, upto the point where a maximum propagation rate is obtained-by injection of substantially `purav Oxygen.

A particularly efcient means for controlling, and particularlyforretarding, the propagation of the combustion `zones in the fractures isthe judicious use of water, especially in liquid form. Water in vaporform is good as a heat-transferring inert gas, and thereforepresaturating` the input gas stream with water vapor is desirable `whenit is needed for slowing down the propagation. Waterin liquid `-forminsmall-amounts, either asslugs or as a continuous addition to theoxygen-containing gas mixture injectedthrouglr the input well, is evenmore. efficient at slowing down the countercurrent combustionpropagation, as quite large amounts of heat can be absorbed andtransferred concurrently by vaporization and condensation., of theliquid water at the temperature levels When slugs of water 6 the.temperature within the fractures may temporarily dropbelow ignitionlevel. However, the interveningperiods between slugs allow al-fverseowof heat from the formations, where the temperature remains high, .backto the cooled fractures to occur in Vorder to re-establish the ignitiontemperature within the fractures for subsequent injection: ofoxygen-containing gases.

Varying the-oxygen'rate alone is a further means of controlling the rateof combustion zone propagation. Combustion and the heat release withinthe fractures occur substantially indirect proportion to the rate ofoxygen injection. Thus a high oxygen input rate means a rapid combustionand release of heat, a high temperature within the fracture, a high rateof diffusion of-heat to the. surrounding yformations, and a relativelyhigh countercurrent propagation rate of the combustion zone along thefracture. The reverse situation is.also true. Low rates of oxygeninjectionmean low combustion and heat release rates, low rate ofdiffusion of heat to the surrounding reservoir, loweredpeak temperatureswithin the fractures, and lower propagation rates of the combustion zonealong the fractures.

Whether the ultimate rise. in temperature that isl obtained within theinterior of:aV given formation is what is desired for the recoveryof agivencil depends upon the balance established between the combustionpropagation rate and the oxygen input rate. The total input of oxygenper square foot of fracture area covered by thecombustion zonepropagation must be such las to supply the total heat required for therecovery of oil from each square foot of the reservoir area `and yallowfor heat losses to the adjacent strata. In determining whether theproper oxygen input and propagation rates are being obtained, heatlosses` to the adjacent strata canabe neglected as a rst approximation,because the main portion of oil recovery can be expected to occurbeforea large fraction of the heatl generated by the combustion crossesthe boundaries of the lreservoir stratum andis. lostrto. the :adjacentstrata.

If, inl order toy obtain the desired temperature rise, it is necessaryto retard the propagation velocity of the combustion zone, the injectionof liquid .water is to be preferred as the most eicient and leastexpensive heat-transferring, propagation-retarding agent. Air is to bepreferred as the oxygen-carrying gas Ibecause it is free. When only asmall retardation of thepropagation rate is desired, the nitrogencontent of the air isusually suflicient to accomplish this. The presenceof the nitrogen, however, does increase the compression costs of theair, but these are ordinari-ly less than the cost of separating thenitrogen from t-he input gas stream.

Referring Iagain to the drawings, the temperature conditions within thestratum 10. after kthe combustion zone has propagated along thefractures 15, 16, Iand 17 fora period of time to the position 20, shownin Figure 1, are qualitatively shown by the graphs of Figure 2. Thethree graphs 21, Y22, and 23 are the temperature profiles or logs as afunction of depth which would Ibe obtained along the lines A-A, B-B',andG-C, respectively, shown in Figure vl. In Figure 2, the indicationsof these logs. or proles-are correlated inposition with the verticalsection yof the stratum 10, illustrated at the left of the gure.r

Log 21, the temperature profile taken along theV line A-A, is thetemperature distribution obtained just after passage of the combustionzone 20. The maximum temperature T2 is that which is found at thelocations ofthe fractures 15, 16, and `17V and is approximately the peaktemperature which occurs within the combustion zone in the fractures.AtV the stratum boundaries and mid-Way between thefractures, thetemperatures shown-by log 21 are only'slightly above the naturaltemperature To of the reservoir.

Log 22, corresponding to a temperature log along the line B-B,represents the conditions existing a substantial period of time afterthe passage of the combustion lZcmfas 20.v A. substantial transfer ofheat by diffusion vertically through the stratum 10 has taken place inthis time interval, as is evidenced by the decrease of the peaktemperatures from the value T2 of log 21, which is accompanied by a risein the temperatures at the stratum boundaries and within the interior ofthe stratum 10 midway within the fractures.

Log 23 represents the temperature distribution across the stratum 10 andthe immediately adjacent strata at the end of a still larger timeinterval following passage of the combustion zones 20. This log showshow the conduction or diffusion of heat through the rock eventuallybrings the temperature throughout the stratum 10 close to an overallvalue of T1. This is the temperature at which the desired modificationsof the oil or hydrocarbon in stratum 10 are obtained to bring about itsrecovery.

Still further characteristics of the invention are shown by the ygraphof Figure 3. This is a plot of the variations of temperature with timeoccurring at a point within a fracture, both as the combustion zoneapproaches, and as it moves past and away from the point. In this caseit is assumed that the combustion zone propagation rate along thefracture is one foot per day. This means, incidentally, that theabscissa scale shown as time in days is also numerically the same as thedistance in feet from the point in question to the combustion zone 20.

At this velocity in a typical medium, lthe temperature rise due toconduction of heat countercurrently through the rock in a horizontaldirection is imperceptible about four days before the arrival of thecombustion zone, or in other words about four feet in front of it. Themajor change in the rate of temperature rise occurs between about oneand two days before the temperature zone arrives, that is, when it isstill between one and two feet away. During the last day and the lastone foot of travel, the temperature climbs rapidly to a value whereignition occurs and heat is released by combustion.

This com-bustion occurs in a relatively short time and space, and theheat so released raises the temperature of the fracture to a high value.This value is difficult to ascertain, as it depends on many factorswhich determine where the balance occurs between the release of the heatin the combustion and its conduction away from the fracture by theadjacent rock. Obviously, the higher the rate of heat release, asdetermined by the consumption of oxygen per unit time, the lhigher willbe the temperature and the balancing rate of heat diffusion through therock of stratum 10.

In Figure 3, the temperature change is shown only in arbitrary runits.Its absolute value depends on many Variables which change itquantitatively, Kbut not qualitatively. After the combustion zone passesthe assumed observation point and has moved on, the fracture temperaturedecreases, at first very rapidly, and then considerably more slowly. Themajor drop of temperature occurs in the first two days, the declinethereafter being more and more gradual.

Still another aspect of the present invention is shown by Figure 4,which presents in greater detail than does Figure 2 the spatialtemperature distribution vertically away from a given point in afracture at several specied times relative to the time of passing of acombustion zone. The conditions for Figure 4 are the same as wereapplicable to Figure 3, namely, that the movement of the combustion zoneis at a velocity of one foot per day. The curve 26, labeled zero days,is the vertical temperature distribution as a function of distance aboveor below the fracture immediately following the passage of thecombustion zone. It is apparent that beyond about 5 feet in a typicalreservoir stratum, the Vtemperature rise is negligible at this time. Thedashed-line curve 27, labeled one day, is the coresponding temperaturedistribution one day later. It is apparent here that a major drop intemperature has occurred at the fracture, but the temperature rise morethan 8 feet away from the fracture is practically negligible.

The solid-line curve 28, labeled ten days, is the temperature profileafter that interval of time, and, as is apparent, a substantial transferof heat has now taken place to distances of the order of 10 feet or so.The dotted-line curve 29, labeled days, shows that over this interval ofdistance and time the temperature has become approximately uniform. Thismeans also that a substantial amount of lheat has been lost to theadjacent formations and that the major portion of the recovery of oilshould have occurred before this time.

As an example of the operation of our invention, it may be assumed to afirst approximation that, if heat losses can be neglected, approximately0.4 cubic feet per day of a stoichiometric mixture of air and fuel gasshould Ibe supplied to each linear foot of fracture zone moving at apropagation rate of one foot per day, for each degree of temperaturerise and each foot of formation thickness. Thus, if a recovery operationin accordance with this invention is being carried out on a fivespotwell pattern and 10-acre well spacing, with an output well at the centerof a square and four input Wells 660 feet away at the corners of thesquare, a simple calculation utilizing this factor will provide aminimum value of gas input rate at any time during the operation. It isonly necessary to multiply together four numbers, one of which is thisfactor of 0.4, and the product obtained is the minimum number of cubicfeet per day of input air and fuel-gas mixture that must be supplied.

Thus, when a combustion front has progressed about half the distance ofa 660-foot well spacing in a tive-spot pattern, so that the length ofthe combustion front is approximately 2,000 feet, if the zone is 30 feetin thickness and is to be heated 200 F. in temperature above the naturallevel, then the volume of input gas mixture in cubic feet per day issimply 0.4 30 200 2,000, which is about 4.8 million cubic feet per day,assuming that the velocity of propagation is controlled to about onefoot per day. Preferably such a formation would be penetrated by severalfractures about equally spaced apart vertically by intervals of 5 to 20feet in thickness.

If the velocity is controlled by inert gas or liquid injection to avalue other than one foot per day, then a corresponding adjustment maybe made in the input gas rate. For example, if the propagation velocityis only 0.5 foot per day, then the input combustible gas rate similarlyshould be divided by two, to become 2.4 million cubic feet per day.

Depending on the number and spacing of the frac tures in the producingformation, this minimum input gas rate may be somewhat increased in anactual case to allow for heat losses to adjacent strata and to insurethat all portions of the formation are heated to at least a certainminimum extent. In general, the closer together the fractures are spacedrelative to the entire thickness of the stratum 10, the closer the inputgas rate can be to the minimum value. This is because with closerspacing the heat diffusion from the fractures to the interveningreservoir rock medium takes place in a relatively shorter length oftime, the oil in place is more quickly recovered, and in this timeinterval a smaller proportion of the heat generatedA is lost to theadjacent formations.

It is within the scope of this invention to alter the gas content of thereservoir stratum 10 before initiating combustion by first holding backpressure on the output well 12 while gases are being injected into theinput well 11. These gases flow along the fractures 15, 16, and 17 withrelative ease and from them permeate the stratum 10 over a wide area. Inpart, the gas may `go into solution in the oil, and when heat issubsequently generated within the fractures in accordance with theinvention, this additional gas saturation within the oil in place, alongwith such gas saturation as naturally occurs in the oil, results insubstantially increased breakout of gas aud formation of bubbles withinthe reservoir rock medium. This substantially increases the forcesavailable to move the liquid oil, with its viscosity lowered by heating,to the fracture and to the output well 12, or directly to the outputwell through the reservoir rock.

By initiating combustion at a relatively high level of static formationpressure and by lowering this pressure during the recovery process, asubstantial increase in percentage of oil recovered is often obtainable,especially in the case of low gas saturation oils. The energy added tothe reservoir oil by increasing the initial static pressure and gassaturation therein is largely recovered by the additional breakout ofgas and the gas drive which takes place upon the occurrence of heating.This is generally a quite efficient way to utilize the driving energy ofgasmuch more so than merely circulating gas through the formation froman input to an output well to entrain or drive liquid through the rockpore spaces by gas 110W alone.

While we have thus described our invention in terms of the foregoingembodiment and specific details, it is to be understood that other andfurther modications will be apparent to those skilled in the art. Theinvention therefore should not be considered as limited in scope to thespecific details set forth, but its scope is properly to be ascertainedby reference to the appended claims.

We claim:

1. In a method of recovering a high viscosity oil from an undergroundproducing formation in which it occurs, wherein heat is generated bycombustion within said producing formation, the improvement whichcomprises the steps of drilling at least two spaced wells into saidformation, one of said wells being an input well and the other of saidWells being an output well; initially providing at least one generallyhorizontal fracture extending continuously through said formationbetween said wells, said fracture having a gas ow capacity which islarge compared to the gas ow capacity of said formation without saidfracture; thereafter initiating combustion at the depth of saidformation within the bore of said output well; injecting a coolcombustion-supporting gas through said input well to ilow primarilythrough said fracture to support said combustion and to propagate a zoneof combustion chiefly along said fracture back toward said input well;controlling the composition and injection rate of saidcombustion-supporting gas so that the equivalent of at least about 0.4cubic foot of a stoichiometric mixture of air and gaseous fuel isconsumed per foot of length of said zone, per foot of propagation, perfoot of thickness of said formation, per degree F. of temperature risenecessary to reduce the viscosity of said oil to a predetermined levelwhere it will flow easily to said output well, whereby the major portionof the heat generation takes place in said fracture, and the oil in saidformation above and below the combusion zone in said fracture is heatedprimarily by the heat diffusing generally vertically through saidformation from said fracture, and whereby the temperature of most ofsaid oil remains substantially below a level where cracking takes place;and recovering from said output well the oil which is released from saidformation by said diffusing heat and which enters said output well.

2. In a method of recovering a high viscosity oil from an undergroundproducing formation in which it occurs, wherein heat is generated bycombustion within said producing formation, the improvement whichcomprises the steps of drilling at least vtwo spaced wells into saidformation, one of said wells being an input well and the other of saidwells being an output well; initially providing a plurality of generallyhorizontal fractures extending through said formation continuouslybetween said wells and through the area of said formation surroundingsaid wells, said fractures being spaced through said formation atapproximately uniform intervals of depth of between about 5 and 20 feet,the lowermost of said fractures being near the lower boundary of saidformation, and the ow capacity of said fractures for gas being largecompare to the gas ow capacity of said formation without said fractures;thereafter initiating combustion at the depth of said formation withinthe bore of said output well; injecting a cool combustion-supporting gasthrough said input well to ow primarily through said fractures to support said combustion and cause zones of combustion to enter andpropagate chiefly along said fractures back toward said input well;controlling the composition and injection rate of saidcombustion-supporting gas so that the equivalent of at least about 0.4cubic foot of a stoichiometric mixture of air and gaseous fuel isconsumed per foot of length of combustion zone, per foot of propagation,per foot of thickness of formation to be heated from each of saidfractures, per degree F. of temperature rise necessary to reduce theviscosity of said oil to a predetermined level where it will flow easilyto said output well, whereby the major portion of the generation of heattakes place within said fractures, and the oil in place in saidformation between and above and below the combustion zones in saidfractures is heated primarily by the heat diffusing generally verticallythrough the formation from said fractures, and whereby the temperatureof most of said oil between said fractures remains substantially below alevel where cracking occurs; and recovering from said output well theoil which is released from said formation by said diffusing heat andwhich enters said output well.

3. In a method of recovering high viscosity oil as set forth in claim 2,the step of adding to said combustionsupporting gas a proportion ofinert heat-transferring fluid effective to retard the propagation rateof said combustion zones through said fractures so that substantiallymore than the equivalent of 0.4 cubic foot of a stoichiometric mixtureof air and gaseous fuel is consumed per foot of length of the combustionzones, per foot of propagation, per foot of formation thickness to beheated by each zone, per degree F. of temperature rise necessary toeduce the viscosity of said oil to a desired predetermined evel.

4. In a method of recovering a high Viscosity oil, steps as set forth inclaim 3 wherein water is injected along with said combustion-supportinggas as said inert heattransferring uid.

References Cited in the tile of this patent UNITED STATES PATENTS1,494,735 Cooper May 20, 1924 2,497,868 Dalin Feb. 2l, 1950 2,584,606Merriam et al Feb. 5, 1952 2,630,307 Martin Mar. 3, 1953 2,693,854Abendroth NOV. 9, 1954 2,754,911 Spearow July 17, 1956 2,780,449 Fisheret al Feb. 5, 1957 OTHER REFERENCES Development of an Underground HeatWave for Oil Recovery, Journal of Petroleum Technology, May 1954, pp.23-33.

1. IN A METHOD OF RECOVERING A HIGH VISCOSITY OIL FROM AN UNDERGROUNDPRODUCING FORMATION IN WHICH IT OCCURS WHEREIN HEAT IS GENERATED BYCOMBUSTION WITHIN SAID PRODUCING FORMATION, THE IMPROVEMENT WHICHCOMPRISES THE STEPS OF DRILLING AT LEAST TWO SPACED WELLS INTO SAIDFORMATION, ONE OF SAIDD WELLS BEING AN INPUT WELL AND THE OTHER OF SAIDWELLS BEING AN OUTPUT WELL; INITIALLY PROVIDING AT LEAST ONE GENERALLYHORIZONTAL FRACTURE EXTENDING CONFINUOUSLY THROUGH SAID FORMATIONBETWEEN SAID WELLS, SAID FRACTURE HAVING A GAS FLOW CAPACITY WHICH ISLARGE COMPARED TO THE GAS FLOW CAPACITY OF SAID FORMATION WITHOUT SAIDFRACTURE; THEREAFTER INITIATING COMBUSTION AT THE DEPTH OF SAIDFORMATION WITHIN THE BORE OF SAID OUTPUT WELL; INJECTING A COOLCOMBUSTION-SUPPORTING GAS THROUGH SAID INPUT WELL TO FLOW PRIMARILYTHROUGH SAID FRACTURE TO SUPPORT SAID COMBUSTION AND TO PROPAGATE A ZONEOF COMBUSTION CHIEFLY ALONG SAID FRACTURE BACK TOWARD SAID INPUT WELL;CONTROLLING THE COMPOSITION AND INJECTION RATE OF SAIDCOMBUSTION-SUPPORTING GAS SO THAT THE EQUIVALENT OF AT LEAST ABOUT 0.4CUBIC FOOT OF A STOICHIOMETRIC MIXTURE OF